Alternative inoculum sources for fire blight: the potential role of fruit mummies and non‐host plants

Fire blight is the most damaging bacterial disease in apple production worldwide. Cankers and symptomless infected shoots are known as sites for the overwintering of Erwinia amylovora, subsequently providing primary inoculum for infection in the spring. In the present work, further potential sources of inoculum were investigated. Real‐time PCR assays covering a 3‐year‐period classified 19·9% of samples taken from fruit mummies as positive. Bacterial abundance in fruit mummies during autumn, winter and spring was up to 10⁹ cells per gram of tissue and correlated well with later infection rates of blossoms. Blossoms of non‐host plants growing close to infected trees were also shown to be colonized by E. amylovora and to enable epiphytic survival and propagation of bacteria. The results indicate a potential role of fruit mummies and buds in overwintering and as a source of primary inoculum for dissemination of the pathogen early in the growing season. Non‐host blossoms may also serve as an inoculum source in the build‐up of the pathogen population. Both aspects may contribute significantly to the epidemiology of E. amylovora. The significance of infected rootstocks as an inoculum source is also discussed. Fruit mummies might be used to determine pathogen pressure in an orchard before the beginning of the blooming period.     

Clove-transmissibility of <Emphasis Type="Italic">Pseudomonas salomonii</Emphasis>, the causal agent of ‘Café au lait’ disease of garlic

Bacterial blight of garlic, caused by Pseudomonas salomonii, results in leaf and sheath necrosis and sometimes leads to soft rot and plant death. The epidemiology of this bacterial disease, known as ‘Café au lait’ disease, is poorly understood and no resistant cultivars are currently available. To develop control strategies for this disease, we investigated principal sources of inoculum. The pathogen was isolated from bulbs from plants with typical vegetative symptoms of bacterial blight. Subsequent development of typical foliar symptoms on plantlets originating from symptomatic bulbs demonstrated transmission of the pathogen in the planting material. In one of three field experiments the contamination rate of planting stock influenced the disease incidence in field-grown garlic. The importance of planting stock as a source of inoculum was demonstrated here and should be evaluated relative to other potential sources such as crop debris, soil or alternate hosts in order to develop successful control strategies.     

Fire blight. Why do views on host invasion by Erwinia amylovora differ?

The fire blight pathogen, Erwinia amylovora, commonly infects flowers and shoots of certain rosaceous hosts and systemic (whole‐tree) invasion sometimes follows. The bacterium may be found in the parenchyma of bark tissue and/or in mature xylem vessels of stem tissue. Views differ on initial sites of multiplication and the optimal route for systemic migration. This article presents the evidence on which the different views are based. There are limited observations on orchard pear and apple trees; in most experimental studies, young apple shoots on potted plants were used. Tissue maturity at the site of shoot inoculation is of prime importance. If xylem vessels are damaged, inoculum may be sucked into the vessels and the bacteria will multiply there. In younger tissue, there is less suction pressure. The critical stem entry site for the invasion of cortical parenchyma seems to be near or above the most recently unfolded leaf. No one has suggested that migration in bark tissue cannot be a major route. If the xylem route is followed, the pathogen needs a means of escape into bark tissue, where typical symptoms develop; means of escape from mature xylem vessels have not been demonstrated and remain a matter for speculation. Published evidence does not seem to support the idea that fire blight is a vascular wilt disease, nor that the extracellular polysaccharide produced by E. amylovora is a toxin and responsible for the wilting symptom seen in the early stages of the disease.     

Detection of Pseudomonas syringae pv. aptata in Irrigation Water Retention Basins by Immunofluorescence Colony-staining

Bacterial blight of cantaloupe (Cucumis melo) caused by Pseudomonas syringae pv. aptata was first observed in south-western France and has since spread to all cantaloupe-growing areas of this country. Use of pesticides registered for this disease has proved ineffective and no commercial cultivars of cantaloupe are resistant to this blight. To develop control strategies for this disease, the principal sources of inoculum were investigated. Among the different sources of inoculum studied, we report the isolation of P. syringae pv. aptata from irrigation water retention basins in south-western France using the immunofluorescence colony-staining (IFC) method. In this study, the pathogen was detected at a low concentration (12 and 70cful^–1) in two different retention basins. These results suggest that P. syringae pv. aptata can survive in water used to irrigate cantaloupe crops and could be a source of inoculum for epidemics of bacterial blight. To develop control strategies for this bacterial disease, the importance of water retention basins as sources of inoculum for bacterial blight of cantaloupe needs to be evaluated relative to other potential sources such as seeds, plants from nurseries and plant debris in the soil.     

The interacting effects of temperature,duration of wetness and inoculum size on the infection of pear blossoms by<Emphasis Type="Italic">Erwinia amylovora</Emphasis>, the causal agent of fire blight

Pear blossoms are the plant tissue that is most vulnerable to infection byErwinia amylovora (Burrill) Winslowet al., the causal agent of fire blight. The interacting effects of temperature, wetness duration and inoculum size on the development of fire blight symptoms in detached pear blossoms were determined in three sets of experiments conducted under controlled conditions. It was expected that this information would facilitate the improvement of a warning system used in fire blight management. Results of the ANOVA tests of the data revealed highly significant interactions among the factors tested. The factors that contributed most to disease incidence were temperature and inoculum size; effects of wetness duration were significant in some cases, but that effect was small. It was further demonstrated that the effects of the interaction of these factors on the incidence of blossom infection may be understood in terms of the general concept of compensation. According to this concept, conditions highly favorable for one of the factors essential for pathogen development may compensate for other factors, for which the conditions are less favorable. As a result of the complex interactions observed between the biotic and abiotic factors, because of compensation relationships and because some of the factors cannot be estimated adequately (for example, inoculum level), it was concluded that it is not yet possible to improve fire blight management by using data on the quantitative relationships between biotic and abiotic factors. Contribution No. 532/02 from the Agricultural Research Organization. http://www.phytoparasitica.org posting Feb. 20, 2003.     

Inoculation of Malus genotypes with a set of Erwinia amylovora strains indicates a gene‐for‐gene relationship between the effector gene eop1 and both Malus floribunda 821 and Malus ‘Evereste’

The Gram‐negative bacterium Erwinia amylovora, causal agent of fire blight disease in pome fruit trees, encodes a type three secretion system (T3SS) that translocates effector proteins into plant cells that collectively function to suppress host defences and enable pathogenesis. Until now, there has only been limited knowledge about the interaction of effector proteins and host resistance presented in several wild Malus species. This study tested disease responses in several Malus wild species with a set of effector deletion mutant strains and several highly virulent E. amylovora strains, which are assumed to influence the host resistance response of fire blight‐resistant Malus species. The findings confirm earlier studies that deletion of the T3SS abolished virulence of the pathogen. Furthermore, a new gene‐for‐gene relationship was established between the effector protein Eop1 and the fire blight resistant ornamental apple cultivar Evereste and the wild species Malus floribunda 821. The results presented here provide new insights into the host–pathogen interactions between Malus sp. and E. amylovora.     

Forecasting the spread of aerially transmitted crop diseases with a binary classifier for inoculum survival

The risk of between‐field spread of disease is typically omitted from crop disease warning systems, as it is difficult to know the number and location of inoculum sources and thus predict the abundance of inoculum arriving at healthy crops. This study explores the utility of a simple approach to predicting risk of between‐field spread, based on the estimated probability that inoculum will survive the transportation process. Using potato late blight as a case study, the effect of solar radiation on the viability of detached Phytophthora infestans sporangia was assessed. A model to estimate the probability of spore survival was derived using a binomial generalized linear mixed model (GLMM), and receiver operating characteristic (ROC) curve analysis and cross‐validation were used to evaluate the global performance of the model as a binary classifier for discriminating between viable and nonviable sporangia. The model yielded an area under the ROC curve of 0.92 (95% CI = 0.90–0.93), signifying an excellent classification algorithm. Inspection of the curve provided a number of suitable decision threshold (or cut‐off) probabilities for discriminating between viable and nonviable sporangia. The classifier was tested as a forecasting system for potato late blight outbreaks using 10 years of outbreak data from across Great Britain. There was a marked differentiation among the cut‐offs, but the best prediction outcome was an accuracy of 89% with an alert frequency of 1 in 7 days. This model can be easily modified or the methodology replicated for other pathosystems characterized by airborne inoculum.     

Colonization of lettuce cultivars and rotation crops by Fusarium oxysporum f. sp. lactucae,the cause of fusarium wilt of lettuce

The severity of fusarium wilt is affected by inoculum density in soil, which is expected to decline during intervals when a non‐susceptible crop is grown. However, the anticipated benefits of crop rotation may not be realized if the pathogen can colonize and produce inoculum on a resistant cultivar or rotation crop. The present study documented colonization of roots of broccoli, cauliflower and spinach by Fusarium oxysporum f. sp. lactucae, the cause of fusarium wilt of lettuce. The frequency of infection was significantly lower on all three rotation crops than on a susceptible lettuce cultivar, and the pathogen was restricted to the cortex of roots of broccoli. However, F. oxysporum f. sp. lactucae was isolated from the root vascular stele of 7·4% of cauliflower plants and 50% of spinach plants that were sampled, indicating a greater potential for colonization and production of inoculum on these crops. The pathogen was also recovered from the root vascular stele of five fusarium wilt‐resistant lettuce cultivars. Thus, disease‐resistant plants may support growth of the pathogen and thereby contribute to an increase in soil inoculum density. Cultivars that were indistinguishable based on above‐ground symptoms, differed significantly in the extent to which they were colonized by F. oxysporum f. sp. lactucae. Less extensively colonized cultivars may prove to be superior sources of resistance to fusarium wilt for use in breeding programmes.     

Intraspecific variation in susceptibility to dothistroma needle blight within native Scottish Pinus sylvestris

Dothistroma needle blight (DNB), caused by Dothistroma septosporum, is the most important disease currently affecting pine plantations in Britain. Intraspecific variation in susceptibility to DNB has been observed in several pine species, but it is not clear if similar variation occurs in Pinus sylvestris (Scots pine), Britain's only native pine. In three separate experiments 2‐ and 3‐year‐old Scots pine saplings from six native Scottish populations were artificially inoculated with D. septosporum conidial suspensions and incubated under conditions optimal for disease development. Conidial suspensions were produced using a single isolate from northeast Scotland. In one experiment, plants were also treated with various spore suspension concentrations to assess the impact of inoculum load on disease severity. There were no significant interactions between host population, plant height, and experiment/inoculum load (anova , P > 0·05), but population, height and inoculum load all significantly affected disease severity (anova , P < 0·05). Among the 2‐year‐old trees, those from Amat were less susceptible than those from Glen Loyne and Glen Cannich (anova , P < 0·05). Among the 3‐year‐old trees, those from Beinn Eighe were less susceptible than those from Abernethy. Plant height and DNB susceptibility had a slightly negative relationship. The use of a spore suspension with a concentration of 1·6 × 10⁶ spores mL^?1 was optimum for disease development. In an in vitro experiment, production of conidia was greater when cultures were incubated in darkness. This paper is the first to report intraspecific variation in DNB susceptibility within Scots pine.     

Spread and development of quambalaria shoot blight in spotted gum plantations

The aim of this study was to examine the disease development of quambalaria shoot blight, caused by the fungal pathogen Quambalaria pitereka, in plantation‐grown spotted gum (Corymbia citriodora subsp. citriodora, C. citriodora subsp. variegata, C. henryi and C. maculata) in south‐east Queensland, Australia. The results showed that native spotted gums are a primary source of inoculum followed rapidly by the production of secondary inoculum from infected trees in the plantation. The rate of spread and development of Q. pitereka within plantations increased exponentially over time as additional trees became infected and produced secondary inoculum. Spore concentration was shown to play an important role in disease development, with disease severity increasing with increasing disease incidence on individual trees and incidence across the plantation.     

Necrotrophic behaviour of Erwinia amylovora in apple and tobacco leaf tissue

An important issue related to the epidemiology of fire blight, a devastating disease of apples and pears, is how its causal agent, the bacterium Erwinia amylovora, survives and disseminates in the environment. Almost no information is available on the possibility of this pathogen overwintering as a necrotroph. In this study, bacterial survival in dead apple and tobacco (a non‐host) leaf tissues was addressed. In necrotized leaves collected 5, 6, 7 and 8 months following shoot inoculation of apple trees, viable E. amylovora cells were present in over 50% of samples from the midrib and in over 10% of samples from lateral veins, but were never found in parenchyma. Using a PCR‐based method, pathogen DNA was detected in more than 50% of samples that were found to be free of viable cells by conventional plating out. However, PCR analysis was insufficient to distinguish between the DNA of viable and dead bacteria. Sugars appropriate for bacterial growth were found in dead apple leaves. In spot‐inoculated attached apple and tobacco leaves, a remarkable increase in the bacterial population was observed in lesions that developed as a hypersensitive response (HR). As in other necrotrophic interactions, bacterial proliferation was associated with massive hydrogen peroxide production and progression toward plant cell death. The results indicate that E. amylovora has an ability to survive as a semi‐necrotroph or necrotroph, which allows for overwintering in dead apple leaves.     

Monitoring the occurrence of tomato bacterial spot and range of the causal agent Xanthomonas perforans in Iran

A 2‐year comprehensive field survey was conducted across major tomato‐growing areas of Iran. Two hundred and thirty‐four tomato fields and six tomato‐producing greenhouses were surveyed for the potential presence of bacterial spot disease. Five hundred and ninety‐six tomato samples with and without symptoms were analysed. While Xanthomonas spp. were found in association with tomato plants both with and without symptoms from five surveyed counties, the bacterial spot disease was observed only in plants from three of them. Only strains isolated from plants with symptoms induced disease symptoms on tomato, while those isolated from symptomless plants caused symptoms only on cabbage and common bean. None of the isolates caused disease symptoms on pepper and eggplant. Phylogenetic analysis showed that X. perforans is the causal agent of tomato bacterial spot in Iran, although X. campestris and X. axonopodis were also associated with symptomless tomato plants. All X. perforans isolates in this study were sensitive to streptomycin, copper sulphate and copper oxychloride at concentrations of 50 mg L^?1, 200 mg L^?1 and 0.8 g L^?1, respectively. Unlike the type strain of X. perforans, isolates in this study did not produce bacteriocin against other Xanthomonas spp., nor were they detected using the usual species‐specific primer pair Bs‐XpF/Bs‐XpR. This suggests an atypical nature of X. perforans strains in Iran, which leads to the hypothesis that X. perforans strains in Iran may have a separate origin to those causing disease epidemics elsewhere. The aggregated dispersal pattern of the diseased tomato fields signifies the seedborne introduction of the pathogen into the country.     

The undiscovered potential of dehydroproline as a fire blight control option

The non‐protein amino acid 3,4‐dehydro‐l ‐proline (DHP) significantly reduced the incidence of fire blight infection on immature pear fruits infected with wildtype Erwinia amylovora. DHP also inhibited biofilm formation in both streptomycin‐sensitive and ‐resistant strains of E. amylovora and induced dispersal of preformed biofilms in the streptomycin‐sensitive strain. The investigations shed light on the hitherto undiscovered ability of DHP to inhibit bacterial biofilms and its potential as a chemical control option for fire blight.     

Effect of flooding on the survival of Leptosphaeria spp. in canola stubble

This study used a versatile temperature‐control device to assess the effect of temperature (12–40°C) and duration (2–12 weeks) of flooding on the survival of Leptosphaeria spp. in canola (Brassica napus) stubble. Canola basal stems with blackleg symptoms were submerged in water in small glass jars containing 20 cm³ soil on a thermogradient plate capable of simultaneously maintaining up to 96 independent temperature regimes. Flooded stems were sampled at 2‐week intervals, surface‐sterilized, and incubated on V8‐juice agar for 10 days to recover the pathogen. Flooding for 2 weeks substantially reduced pathogen recovery relative to non‐flooded controls and the pathogen was not recovered after 6 weeks of flooding, irrespective of temperature. The pathogen was eliminated slightly more rapidly at flooding temperatures >20°C than at 12–16°C. There was no difference between Leptosphaeria maculans and L. biglobosa in their ability to survive flooding. Stem tissues degraded most rapidly during the first 2 weeks of flooding, corresponding to a quick decline in pathogen survival during the same period. These results indicate that a paddy rice crop following winter rapeseed may minimize the impact of blackleg by eradicating the inoculum of Leptosphaeria spp. in stubble.     

Temporal development and relationship amongst brown rot blossom blight,fruit blight and fruit rot in integrated and organic sour cherry orchards

The aim of this 4‐year study was to characterize temporal development of brown rot blossom blight and fruit blight (caused by Monilinia spp.) and their sporulating areas in sour cherry orchards; and to determine the relationships amongst incidence and sporulating area of blossom blight, fruit blight and fruit rot. The study was performed in integrated and organic orchard blocks on two cultivars (Újfehértói fürtös and Érdi b?term?). On both cultivars, disease progress on flowers and fruits was 2–10 times slower in the integrated than in the organic management system. The peak incidence values were 9 and 31 days after petal fall for blossom blight and fruit blight, respectively. After these dates, no new blight symptoms on flowers and/or fruits appeared and the disease was levelling off. Final blossom blight incidence ranged from 1 to 5% and from 12 to 34%, and fruit rot incidence from 2 to 6% and from 11 to 26% in the integrated and the organic orchards, respectively. The sum of fruit blight incidence ranged from 9 to 22% for the organic system, but was below 5% for the integrated system, while the final sporulating area was 5–16 mm² and <3 mm², respectively. Among the five highest Pearson's correlation coefficients, relationships between blossom blight and early fruit blight stage (r = 0·845, P = 0·0087 integrated; r = 0·901, P = 0·0015 organic), and between sporulating area and fruit rot (r = 0791, P = 0·0199 integrated; r = 0·874, P = 0·0039 organic) were the most significant relationships from an epidemic standpoint as they indicated a connection between different brown rot symptom types.     

Management of bakanae and bacterial seedling blight diseases in nurseries by irradiating rice seeds with atmospheric plasma

The control of seedborne rice seedling diseases in the seed beds is important to avoid epidemics in rice nurseries and paddies, which may result in severe yield loss. Recently, irradiation with plasma containing electrons, creating positive or negative ions and neutral species, has been shown to have an antimicrobial effect, probably via generation of reactive oxygen species. This study examines whether two seedborne rice seedling diseases, bakanae disease caused by the fungal pathogen Fusarium fujikuroi, and bacterial seedling blight caused by Burkholderia plantarii, are suppressed by irradiation of infected rice seeds with atmospheric plasma. Seed germination and seedling growth were not inhibited in plasma‐treated healthy seeds. When F. fujikuroi‐infected rice seeds were irradiated with plasma after being immersed in sterile distilled water, bakanae disease severity index and the percentage of plants with symptoms were reduced to 18.1% and 7.8% of non‐irradiated control, respectively, depending on the duration of plasma irradiation. The bacterial seedling blight disease index was also reduced by plasma irradiation in vacuum‐inoculated seeds to 38.6% of the non‐irradiated control, and in infected seeds harvested from spray‐inoculated heads of rice plants to 40.1% of the control. Therefore, plasma irradiation seems to be effective in controlling two independent seedborne rice seedling diseases.     

Effect of initial darkness duration on the pathogenicity of Calonectria pseudonaviculata on boxwood

Light is an important environmental stimulus that regulates many physiological activities within plants and pathogenic fungi. Boxwood blight (causal agent: Calonectria pseudonaviculata) is a newly emergent disease in the United States and a significant threat to the boxwood industry. A disease‐forecasting model has been developed to aid boxwood growers in their management decisions, but light is not included as a variable within the programme. Growth chamber experiments were conducted to evaluate the effect of an initial dark period on disease severity, mycelial growth and conidial germination of C. pseudonaviculata. Treatments varied by darkness duration (0, 6, 12 or 24 h dark), and were followed by a 14 h day length at a 25/21 °C day/night temperature. Neither boxwood blight severity nor fungal colony development was enhanced by an initial dark period. However, darkness duration was associated with a significant increase in conidial germination in vitro. This study suggests the predictive performance of the current boxwood blight disease‐forecasting model would not be improved by inclusion of photoperiod.     

Spatiotemporal analyses of rhizopus rot progress in peach fruit inoculated with Rhizopus stolonifer

Rhizopus rot, caused by Rhizopus stolonifer, is one of the main postharvest diseases in stone fruits, but there is little known about the processes of disease development during transport and postharvest storage. The objective of this study was to characterize temporal progress and spatial distribution of the disease in peach fruit. Rhizopus rot development was evaluated using two different fruit arrangements. Only one fruit of each arrangement was inoculated with a R. stolonifer spore suspension. Disease incidence and severity were assessed daily for all the fruit. Nonlinear models were fitted to the quantity of fruit and to the area of fruit that became infected over time and distance in relation to the source of inoculum. Disease‐free fruit placed next to the artificially inoculated peaches showed disease symptoms due to pathogen dissemination by mycelial stolons. The disease incidence and severity progress rates varied from 0.33 to 0.53 day^?1 and from 0.30 to 0.49 day^?1, respectively. The spatial spread of the disease followed a dispersive wave pattern with increasing speed over time, but decreasing speed with disease severity. For disease severity y = 0.5, the velocity at day 3 varied from 0.14 to 0.32 fruit diameter day^?1, while it ranged from 0.38 to 1.46 fruit diameter day^?1 at day 12.     

Effects of Environmental Conditions on the Development of <Emphasis Type="Italic">Fusarium</Emphasis> Ear Blight

Recent research on the epidemiology of Fusarium ear (or head) blight (FEB or FHB) of small-grain cereals is reviewed, focusing on inoculum, infection and disease forecasting. Both conidia and ascospores have been shown to be important for causing FEB. For Fusarium graminearum, propagules from crop debris are the main source of initial inoculum. Inoculum production is critically dependent on rainfall although the precise relationship is not clear. Recent work on understanding the effects of climatic variables on FEB development has been based on field observations. These field-based studies confirmed that warm and moist conditions during anthesis are the key factors for FEB development. Several empirical models were derived from the field data and proposed for use in disease forecasting. However, these models may not be applicable to a broader range of areas because of the limited nature of the field data. Several areas are proposed for future research, focusing on the development of more generally applicable forecasting models and on understanding the relationships between disease severity, fungal biomass and the production of associated mycotoxins.     

Eucablight – collating and analysing pathogenicity and resistance data on a European scale*

The Concerted Action on potato late blight ‘Eucablight’ ( http://www.eucablight.org ) was launched in 2003 to promote collaboration between researchers across Europe and to collate the previously fragmented data on host resistance to late blight and characterization of populations of its causal agent, Phytophthora infestans. Fundamental to the project was the development of standardized protocols, databases and data collection tools with which data from across Europe could be centralized. The ultimate aim was to capitalize on the vast resource of data available in order to allow a pan‐European analysis to be conducted. The P. infestans database is currently populated with information relating to over 15 000 isolates from 20 European countries. The host resistance database holds primary disease data, and derived statistics, from more than 50 field trials assessing mainly foliage blight resistance. The use of five standard cultivars in these trials facilitated the comparison of resistance information across years and regions. The host database is structured, and made accessible, in such a way that DSS builders can access the model parameters they need to construct locally adapted forecasting systems. In this presentation we describe the database and the data analysis tools that have been developed in this project and how they are directly applicable to other host‐pathogen systems.